The Royal Australian Navy has conceded there might be design faults with its two largest ships which have been docked in Sydney since March undergoing urgent repair workEngineers are still trying to identify what is causing problems with the "azimuth" propulsion system on board the $1.5 billion Landing Helicopter Docks (LHDs) HMAS Canberra and HMAS Adelaide.

"It may well be a design issue," conceded Rear Admiral Adam Grunsell, the head of maritime systems in Defence's Capability Acquisition and Sustainment Group.The diesel-electric powered propellers which sit at the stern of the LHDs are known as "azi-pods" and are mounted on steerable pods that have a 360-degree rotation.

Preliminary samples taken from the propulsion pods on board HMAS Adelaide have identified metal fragments in lubricants, while faulty engine seals on HMAS Canberra are believed to be responsible for the "migration", or leaking, of various oil types into different engine areas.

Asked whether the problems could be due to a design fault or because the ships have been operating at sea for too many days, Chief of Navy Vice Admiral Tim Barrett said it was too early to say.

"I would not speculate on any of those outcomes, but clearly in a root cause analysis you have to consider all of those options," he said